It's actually quite common behavior.
But Obama campaigned on changing every aspect of such things, and said that he would guarantee the most transparent administration in history. And here we have a person that he trusts enough to put in the line of succession to his office (Secretary of State) that - on being nominated - didn't just flub her way through a crappy email backup system (a la the career IT people in the WH during Bush, which were not appointees - these are permanent staffers, which you do understand, right?), but rather she immediately went about setting up a system to prevent her communications from being part of the official record.
Then she went around the world doing things like posing with giant plastic "reset" buttons to make everything wonderful with Russia and whatnot, even as she was soliciting millions in donations from foreign governments for use by her personal family foundation. But we'll never know what those emails looked like, and how such things might have been tied to or tangled up with her official duties, because she shielded all of those messages from FOIA requests by never having an official box. And when pressed, she had her own loyalists go through some of the message, and pass along those that SHE considered appropriate for the public archive.
Completely pre-meditated obfuscation of her communications as a senior official. No Sarah-Palin-style cluelessness about using her Yahoo account, no career IT people in the White House having a lame backup system ... no, the completely planned in advance absence of any records except those that Clinton decided, later, should be present. Today we see reports that the IT people in the State Department warned her that her not having an official State mailbox was going to endanger compliance with record keeping laws, but that her completely casual personal mail server was a huge, huge security risk. So we have not only premeditated law breaking to avoid transparency and accountability, but we also have horrible incompetence in understanding the risks of conducting top-level international diplomacy via a mail server set up by some guy with a fictitious name, paid in cash. One really can't make this stuff up.